The present invention relates to the preparation of strontium carbonate by the reaction of strontium nitrate with carbon dioxide.
Strontium carbonate is used in the manufacture of television picture tube face-plate glass, as an x-ray screening agent, and in ferrite magnets.
Most strontium carbonate is derived from natural ores, usually celestite (predominantly strontium sulfate).
The "black ash" process is the commercial process currently employed for producing strontium carbonate from celestite ore. Ground celestite is mixed with finely divided carbon and the mixture calcined at a temperature of about 1000.degree. C. to yield a black ash containing strontium sulfide (SrS) from the strontium sulfate in the ore and gaseous by products carbon dioxide (CO.sub.2) and carbon monoxide (CO). The water-soluble strontium sulfide is leached from the black ash with hot water, and the aqueous solution then separated from the undissolved ore impurities by settling and filtration. The dissolved strontium sulfide is converted to strontium carbonate by carbonating the aqueous solution with CO.sub.2 or, more commonly, by reacting the aqueous solution with sodium carbonate.
The strontium carbonate precipitates from solution and is recovered by conventional solids-liquid separation procedures and dried.
Disadvantages of the black ash process include the requirement of high process temperatures and disposal of the reaction byproducts, sodium sulfide (when sodium carbonate is the carbonate source) or hydrogen sulfide (when CO.sub.2 is the carbonate source).
The strontium carbonate produced by the black ash process may also be converted to other strontium salt products, e.g., strontium nitrate. Strontium nitrate is typically prepared by dissolving strontium carbonate with a strong aqueous nitric acid solution, thus forming strontium nitrate and carbon dioxide. Impurities in the resultant aqueous solution are precipitated with lime and removed, before strontium nitrate is recovered from the filtered solution by evaporative crystallization.
The process of this invention provides an alternative to the production of strontium carbonate via the conventional black ash process, by employing strontium nitrate as a strontium source. Such strontium nitrate may be prepared from celestite ore by the process described in our co-pending U.S. application Ser. No. 273,282, filed June 12, 1981, now U.S. Pat. No. 4,337,233, which process may advantageously be integrated with the present invention.